1. Field of the Invention
The invention relates to automatic vehicle transmissions with stepped ratios.
2. Discussion of the Background
In most automatic transmissions with stepped ratios the changes of ratio in automatic operation are decided in accordance with criteria that notably include taking into account the speed of the vehicle and the engine loading. More often than not these criteria are translated into the form of ratio change laws which are conveniently visualized in the Vv/.alpha.c coordinate plane where Vv designates the vehicle speed and .alpha.c the opening of the member regulating admission of fuel to the engine. It is constantly the case that these laws are defined in such a way that the ratio changes are made at higher vehicle speeds the higher the engine loading. Furthermore, and also constantly, the N-1/N change, i.e. shifting up from a lower ratio to a higher ratio, is performed for the same engine load, or more exactly for the same .alpha.c opening at a speed which is significantly higher than the speed at which the reverse change N/N-1, i.e. the down shift, takes place. This arrangement, which can be considered as a hysteresis effect, is designed to prevent an unacceptable tendancy to hunt between the N-1 and N ratios which would be bound to occur if the up and down shift were performed under the same speed and load conditions.
As an example FIG. 1 shows a typical set of speed change laws for a three-ratio automatic transmission. A look at this diagram shows that the corresponding change laws are insufficient and unsuitable in certain configurations although they are very commonly met when driving an automobile. If, for example, we consider a vehicle in an acceleration phase in second with a virtually constant accelerator position .alpha.c, in FIG. 2 this phase is shown by a segment oriented as AB. If we now assume that at point B the driver is forced to suddenly lift his foot off the accelerator this operation appears in this same FIG. 2 as the straight line segment BC, roughly vertical if the release of the accelerator is so fast that the vehicle speed has no time to fall. It can then be seen that if path BC crosses the 2/3 change curve the transmission goes automatically into third. But in most cases this ratio change is quite inopportune, since the engine braking of the vehicle is diminished by the change into third even though the driver explicitly expressed his intention to slow down by quickly lifting his foot off. All drivers of vehicles fitted with this automatic transmission know this typical behaviour well and generally complain about it. In certain cases resorting manually to imposed speed positions enables this defect to be remedied in part but, by definition, it does not involve automatic intervention of the transmission control system and also, in an emergency the driver does not have the material time available to use the imposed positions.